The present invention relates to a full color illuminating unit many of which are installed on a plate thus forming a dot matrix display and, more particularly, to an improved full color illuminating unit comprising a blue light positioned in a first physical level and a plurality of red lights and green lights positioned in a second physical level around the blue light.
Conventional dot matrix displays, which employ red, green, and blue light emitting diodes (LED), are becoming more and more popular in advertising. The blue light emitting diodes, in addition to low emitting efficiency, are expensive and difficult to manufacture. The dot matrix display is limited to be used indoors since the brightness thereof is insufficient for outdoor use. Furthermore, the LED type dot matrix display, when used commercially, merely uses red and green LEDs upon economic consideration, and is thus unable to generate all kinds of colors. To eliminate the drawbacks of light emitting diodes, light bulbs of cathode ray type have been used for outdoor advertising. Yet the cathode-ray type light bulbs generate considerable heat and results in heat dissipation problems and affecting the life thereof. The inventor of the present invention has invented a full color dot matrix display and filed a foreign application with a serial number 83842/4. Referring to FIG. 4, the 83842/4 application discloses a full color dot matrix display which comprises a housing 90 including a front panel 81 and a rear plate 82, a first circuit board 96 and a second circuit board 97 provided between the front panel 81 and the rear plate 82, and a plurality of illuminating units 9 arranged in the form of a dot matrix and is mounted on the first circuit board 96 and the second circuit board 97.
Further referring to FIG. 4 and FIG. 5, each illuminating unit 9 comprises a substrate 91, a central blue light bulb 94 of cathode ray type mounted in a central hole 92 in the substrate 91, and a plurality of red and green light emitting diodes 93 (LEDs) mounted around the blue light bulb 94. The LEDs 93 are electrically connected to the first circuit board 96 by pins 95 on the substrate 91. The blue light bulb 94 extends beyond the substrate 91 through a hole 220 in the first circuit board 96 and electrically connects to the second circuit board 97.
However, there are some drawbacks existing in the above example. Firstly, the color combination can not achieve uniformality in some angular ranges. This is because the blue light from the cathode ray type bulb is visible in an angular range up to 75.degree. with respect to a longitudinal central line thereof, yet the light (either red and green) from the LED is visible in another angular range up to 30.degree. as illustrated in FIG. 6. For the above reason, the combination light is not uniform in an angular range from 30.degree. to 75.degree. which is called "non-uniform range". Therefore, any person who stands in the "non-uniform range" will see blue light only. Secondly, the blue light bulb 94 is firmly confined between the first circuit board 96 and the second circuit board 97, thus it is cumbersome to replace a broken blue light bulb 94 in this structure. Unfortunately, the blue light bulb 94 normally has two to three year life time. Thirdly, the display housing 90 is normally installed outdoors and is apt to be splashed by rain, thus any of the illuminating units 9 might get wet and the bulbs therein might be damaged.
Therefore, there has been an unfulfilled need for an improved full color dot matrix display to mitigate and/or obviate the aforementioned problems.